Photographic cameras and video cameras are equipped with autofocus systems by which a taking lens can be automatically adjusted to the proper focus for a subject. An autofocus system is constructed of a range finding device for detecting the distance to a subject and a lens setting mechanism for positioning the taking lens in accordance with the detected subject distance. Various types of range finding devices are known. Among these, an active triangulation type is most widely used. Such an active triangulation type range finding device is comprised by a light projector for projecting a spot of light toward a subject, a light receiver for receiving light reflected from the subject, and an arithmetic operation unit for calculating the subject distance in accordance with a signal from the light receiver and determining the setting position of the taking lens in accordance with the calculated subject distance. As a light source of the light projector, an IRED (near-infrared light emission diode) for emitting near-infrared light is mainly used. As a light receiving element of the light receiver, a PSD (position sensitive detector) is mainly used which generates two channel signals corresponding to the incident position and intensity of the reflected light.
In order to emit near-infrared light of sufficient intensity for a relatively far subject, a mirror with a parabolic reflective surface is disposed behind an IRED, and near-infrared light emitted obliquely from the IRED and reflected by the mirror is projected toward the subject. For example, a light projector disclosed in Japanese Utility Model Laid-Open Publ. No. 59-109165 has a mirror 71 surrounding an IRED 70 as shown in FIGS. 10 and 11. Near-infrared light 72 radiated obliquely from the IRED 70 is reflected forward by the parabolic reflection surface 71a of the mirror 71. With this light projector, not only near-infrared light 73 directly incident on a light projecting lens 74 but also near-infrared light 72 incident on the light projecting lens 74 after having been reflected by the mirror 71, are used in range finding. Therefore, the intensity of measuring light 75 is sufficient to measure greater subject distances.
However, as shown in FIG. 12, the pattern of near-infrared light projected toward a principal subject S is composed of a central area 75a and a peripheral annular area 75b surrounding the central area 75a. One side of the central area 75a has a length of 40 to 50 cm at a subject distance of about 8 meters for example. It often occurs therefore that the peripheral area 75b extends partially beyond the subject S.
On the other hand, a light pattern 90 is formed on the light receiving surface of a PSD 79 as shown in FIG. 13. Since the direction of range finding resolution of the PSD 79 is in the base line direction X, the ratio between two channel signals outputted from the PSD 79 has a value equivalent to the value when a near-infrared spot of light is incident upon the center G.sub.1 of gravity of the central area 90a and the peripheral area 90b of the light pattern 90. The equivalent light incident position is therefore displaced by a length D from the correct incident position, and this is an important cause of range finding errors.
With a range finding device which projects a single spot of light onto the center of a photographic scene, the spot of light can pass between two persons when they are standing at a distance from each other, so that the taking lens is focussed on infinity. With a recent range finding device disclosed for example in U.S. Pat. Nos. 4,470,681 and 4,571,048, three spot beams of light are sequentially projected in the lateral direction of a photographic scene; the subject distance is measured for each spot beam of light, and the most suitable one of the three measured distances is selected. As a light projector for such a multi-beam range finding device, there is used a light source unit having a plurality of light sources. An example of such a light source unit 95 is shown in FIG. 14 and has a plurality of IREDs 92, 93 and 94 mounted on a base plate 96 for the emission of near-infrared light, these IREDs are sealed within a transparent molded plastic package 97. The light source unit 95 is fitted within a recess 91 formed in the camera body to hold the light source unit 95 in position. Similarly, a PSD 79 used as a light receiver with this light source unit 95 is sealed within a transparent package to hold the light receiver in position using the outer contours of the package.
This mounting method however has a limit of positioning precision of the IREDs 92 to 94 in the order of 200 microns at most because of the molding precision of the light source unit 95 and the working precision of the recess 97. For this reason, as shown in FIG. 15, it often happens that the alignment line P of the IREDs 92 to 94 cannot be set correctly relative to the base line direction X such that the angle .theta. is 90.degree.. If near-infrared light beams are reflected by a subject having parts at the same object distances, such as a wall, then near infrared light beams 92a, 93a and 94a incident on a PSD 79 are disposed on a line 98 which is inclined at the same angle as the alignment line P of the IREDs 92 to 94. In such a case, the distance data calculated for each near-infrared light beam 92a, 93a, 94a become different, which is a substantial cause of range finding error. Furthermore, if the PSD 79 is correctly aligned so as to match the direction of range finding resolution in the base line direction X, a range finding error will be generated for the same reason described above.